Chromatography is a method of separating mixtures of compounds into their components. It enables one to separate trace impurities or major fractions from each other. These separated components can then be analyzed by various methods, including spectrographic methods.
Chromatography is based on the separation of different types of molecules as they pass down along a column which contains material that exhibits attractive selectivity for certain compounds. The column may be packed with a material that provides a high surface area or a wall coated film, and that interacts physically with the compounds being analyzed. The packing or wall coated film of inert material acts as a stationary phase. As the sample passes along the column, it separates into its different components which can then be characterized and identified. This can also be used to measure how much of each component is present in the mixture.
One particular type of chromatography is gas chromatography. In gas chromatography, the sample is transformed into a gas phase. An inert gas is used as a carrier to propel the sample along the column under constant mass flow rate conditions. The inert gas passes through the chromatographic column at a constant velocity because it does not interact with and therefore spend time on the stationary phase. The sample is injected into the carrier gas and is swept along the chromatographic column. The different substances or analytes within the sample interact differently with the stationary phase, thus taking different times to pass through and exit the column to reach the detector. This permits independent analysis of the analyte components.
In order to provide greater and greater sensitivity, the diameter of the column has been decreased and its length has been increased. Very narrow columns (0.1 mm-0.5 mm internal diameter) are used. These are referred to as capillary columns and they may be several hundred feet long. These are not packed, but rather the inside wall of the column is coated with a selective partitioning material. These allow sample mixtures dissolved in 1 .mu.l-5 .mu.l of solvent to be injected and analyzed.
With these capillary column gas chromatographs, the solvent becomes an overwhelming problem. The sample size simply needs to be minimized in order to permit proper separation due to the lower mass loading capacities of these films. Where the substances being detected are at very low concentrations, this can present a problem. There simply may not be enough of the analyte in the solvent to be properly detected.
One way to resolve this issue has been to inject a larger sample and separate the solvent from the sample after it is injected. One particular process is programmed temperature sample introduction, also known as "PTV injection." In PTV injection, the sample with the solvent is injected into a liner which serves as an evaporator chamber. The temperature of the tube can be varied from relatively low temperature causing the sample to remain in the liner, then heated to provide a gas phase mixture which is now concentrated within the range of detection. The carrier gas in introduced through the tube, causing evaporation of the solvent. Generally the solvent is more volatile than the sample and is allowed to be vented into the purge of the gas chromatograph. In order for this to effectively concentrate a sample, the sample must be injected at a rate which matches the solvent evaporation rate. This requires a very expensive programmable injector. Typically, samples are introduced into a gas chromatograph by simply pressing a hypodermic syringe manually. The need to rely on a programmable injector is time-consuming and, in addition, significantly increases capital expenses.
Further, with the PTV apparatus, the purge and the carrier gas flow are not independent of the gas chromatograph. Thus, the purge and carrier gas flow are optimized to meet the requirements of the gas chromatograph. This all has a limiting effect on the amount of solvent that can be split from the sample. The PTV has limited geometry (typically 80 mm in length) and thus cannot serve as a chromatographic device, but as an injection device only.